MEMS are miniaturized devices consisting of one or more micro machined components or structures. These devices are usually fabricated through the integration of several components with diversified functions such as mechanical, electrical, magnetic, biological etc. . . . functions. In the past two decades, many MEMS devices have been successfully put into applications, although the majority of MEMS devices are still under research or investigation. MEMS device research and development is rapidly emerging, and now extends to nearly every industrial field including automotive, bio-engineering/biomedical, telecommunications, electronics, space, military and gaming industries, etc. . . . [Marinis 2009].
In general, despite ongoing research and development activities, the reliability of MEMS devices needs significant improvement. One of the major issues limiting the reliability of MEMS devices is tribology, specifically friction, adhesion and wear. If two non-lubricated MEMS surfaces come into contact, they show high adhesion force, high friction and eventually lead to wear (loss of material). Hence, if these tribological issues are not addressed properly, they limit the operating life of MEMS devices and eventually their reliability [Kim et al 2007].
As the size of the MEMS devices is measured in micro-meters (or sub-millimeters), the surface-to-volume ratio is very high when compared to the gravity and inertial forces, and hence surface forces such as van der Waals, capillary, electrostatic, and chemical forces play an important role in device performance. Hence, in these devices, the tribological and the interfacial forces are comparable to or higher than the forces causing device motion. Hence, there is a great need for the solutions which address tribological issues in MEMS devices.
The mainstay structural material from which MEMS devices are made has been Si, particularly as a result of the mature Si material fabrication capabilities (arising from integrated circuit/microchip industry) at the micro-meter scale. However, Si does not have good tribological properties and it shows high friction, adhesion force and wear, respectively, when sliding against itself or any other material. Therefore, thorough research was conducted over the past two decades in an effort to improve the tribological properties of Si. Because of the small length scales of MEMS devices, macro scale oil based lubrication methods are not applicable and hence researchers have developed certain specific types of nano-meter to sub-micrometer thick films (mostly solid lubricants) which reduce friction, adhesion force and wear, respectively. These thin films include self-assembled monolayers (SAMs), polymer coatings, vapor deposited organic layers, fluorine based organic layers, solid coatings etc [Patton et al 2007 & 2008; Knieling et al 2007; Henck 1997; Satyanarayana and Sinha 2005; Eapen et al 2005; Ma et al 2007; Sidorenko et al 2002 (a); Satyanarayana et al 2009; Asay et al 2008 (a) & (b); Lee et al 2005; Prasad et al 2009 and Scharf et al 2006]. Though Si has several advantages, it has certain inherent drawbacks such as brittleness, high friction and adhesion force, non-biocompatibility etc. . . . Therefore, recently, SU-8 has been replacing Si for certain applications [Abgrall et al 2007].
SU-8 polymer is an epoxy-type UV-sensitive negative photoresist material which is used as a photoresist as well as a structural material in the fabrication of MEMS. SU-8 has several advantages such as ease of fabrication, hydrophobic nature, high thermal stability, good chemical resistance and biocompatibility which makes it an attractive material from which to fabricate MEMS devices. Despite its advantages, SU-8 has some disadvantages such as poor mechanical and tribological properties, respectively, and high internal stresses which limit its use as a structural material for fabricating reliable MEMS structures with complex designs and multiple functions.
Jiguet et al [2006 (a) & (b)] have studied the effect of the addition of silica nanoparticles (diameter: 13 nm and concentration: 5 wt %) to SU-8, and the effect of thermal treatment on the friction and the wear properties of SU-8. Sliding tests were conducted against steel and POM (polyoxymethylene) balls. The SU-8 nanocomposites reduced wear rates and friction coefficients marginally when compared to the un-reinforced SU-8. The improvement was very minimal when compared to the stringent property requirements of MEMS devices in terms of their reliability, longer operating life time and accuracy. This study has also deduced that the coefficient of friction of the composites depends on the counterface material and the elastic properties of the SU-8 material. This study has also shown that heat treatment can considerably reduce the wear rate of reinforced and un-reinforced materials.
Singh et al [2011 (a)] have developed a two-step SU-8 surface modification method (i.e., modification of an outer surface of an SU-8 structure) which has improved the tribological properties of SU-8 film surfaces by several fold. The two-step surface modification consists of first treating an SU-8 film surface with oxygen plasma, followed by the application of a nanolubricant such as PFPE. By the application of the two-step surface modification method to SU-8 thin (thickness: 500 nm) and thick films (thickness: 50 μm) on Si surfaces, the initial coefficient of friction has been reduced by ˜4-7 times, the steady-state coefficient of friction has been reduced by ˜2.5-3.5 times and the wear durability has been increased by >1000 times. The authors have done the tribological tests under the loading conditions of a normal load of 150 g and a rotational speed of 200 rpm. The two-step surface modification method has slightly reduced the elastic modulus and hardness of pristine SU-8 thick films, as observed in nanoindentation tests.
Singh et al [2011 (b)] have also developed a two-step SU-8 chemical modification method (i.e. modification of an outer surface of an SU-8 structure) which has improved the tribological properties of SU-8 film surfaces by several fold. The two-step chemical modification method consists of first chemically treating an SU-8 film using ethanolamine-sodium phosphate buffer, followed by the coating of PFPE nanolubricant. By the application of the two-step chemical modification method to SU-8 thin (thickness: 500 nm) and thick films (thickness: 50 μm) on Si surfaces, the steady-state coefficient of friction has been reduced by ˜4-5 times and the wear durability has been increased by >1000 times. The authors have done the tribological tests under the loading conditions of a normal load of 150 g and a rotational speed of 200 rpm. The authors have attributed the significant reduction in the friction coefficients to the lubrication effect of PFPE nanolubricant, while the exceptional increase in their wear life was attributed to the bonding between the —OH functional groups of ethanolamine treated SU-8 thin/thick films and the —OH functional groups of PFPE.
Voigt et al [2007] have demonstrated the miscibility of epoxy resin surface-modified SiO2 nanoparticles (average particle diameter: 20 nm) into epoxy photomaterial to create a photo-patternable material with improved lithographic, optical and mechanical properties. The addition of the epoxy resin surface-modified SiO2 nanoparticles to epoxy resin has increased the Young's modulus as observed in the nanoindentation tests and it was observed that the Young's modulus has increased with the content of the SiO2 particles. For the highest silica content added, the Young's modulus has increased from 5.8 GPa to 8.7 GPa.
Chiamori et al [2008] have investigated the mechanical properties of SU-8 composite materials added with diamondoids and SWCNTs (single-walled carbon nanotubes) (average diameter: 0.8 nm and concentration: 1 wt % and 5 wt %). Uniaxial tensile tests were conducted on nanocomposite samples using a MTS Bionix 200 tensile tester and the effective elastic modulus was extracted from the force-displacement curves and geometry of the samples. SU-8 has shown an elastic modulus of 1.6 GPa whereas the diamondoid and SWCNTs added SU-8 has shown elastic modulus values of 1.9 GPa and 1.3 GPa, respectively. In their study, the addition of the nanoparticles did not show any significant improvement in the mechanical properties of SU-8 (SWCNTs have in fact slightly reduced the elastic modulus of SU-8).
Mionic et al [2010] have fabricated SU-8+MWCNT (multiwalled carbon nanotubes) (concentration: 5 wt %) composites and measured their mechanical properties using nanoindentation. They have also studied the influence of SU-8 solvent on the structural homogeneity and mechanical properties of the composites. They have observed that the solvent type and the functionalization of MWCNTs affect the Young's modulus of the composites. In their study, a highest increase of the Young's modulus of 104% in respect to the parent material was observed when acetone was used as the solvent.
From the above studies it is clear that researchers have developed different strategies in their attempts to improve either tribological properties or mechanical properties of SU-8. That is, the above studies have demonstrated independent improvement in tribological properties of SU-8, without improvement in mechanical properties of SU-8; or independent improvement in the mechanical properties of SU-8, without improvement in tribological properties of SU-8.
Liquid lubricants have been added to certain polymer nanolayers [Julthongpiput et al 2002 (a) and (b), Sidorenko et al 2002 (b) and Ahn et al 2003] and also to other polymers such as high density polyethylene (HDPE), polyolefin and ultra-high-molecular weight polyethylene (UHMWPE) [Puukilainen et al 2005, 2006, 2007]. In research works conducted by Tsukruk and co-workers [Julthongpiput et al 2002 (a) and (b), Sidorenko et al 2002 (b) and Ahn et al 2003], paraffin oil (C15H32—C24H50) was added into a polymer nanolayer of poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS), and the enrichment of paraffin oil into the polymer nanolayers improved wear durability. Whereas, in the research works conducted by Puukilainen and co-workers [Puukilainen et al 2005, 2006, 2007], they have blended PFPE into polymers such as HDPE, polyolefin and UHMWPE and the blending of PFPE into these polymers has improved hydrophobic property and the tribological properties (i.e., lowered the coefficient of friction and increased abrasion resistance).
A need clearly exists for a manner of simultaneously improving the tribological and mechanical properties of SU-8 or other materials that can be used to produce MEMS components or devices.